A vertical reciprocating conveyor is employed to move cargo between two or more different vertical levels.
The typical vertical conveyor includes a carriage having a platform to support the cargo and the carriage is guided for vertical movement between the lower and upper levels on a supporting structure or frame that includes a pair of spaced vertical columns. With a hydraulically actuated conveyor, one or more hydraulic cylinder units are each connected through a cable or chain mechanism to the carriage and through extension of the cylinder units, the carriage can be raised to the upper level. To lower the carriage, operation of the pump motor of the hydraulic system is terminated and valving is actuated to permit discharge of the hydraulic fluid from the cylinder units and consequent controlled lowering of the carriage.
A typical mechanically operated vertical conveyor utilizes an electric motor that is connected through a chain drive to the carriage. Operation of the reversible motor will act to raise and lower the carriage.
As a safety feature, the typical vertical conveyor includes an overload protection mechanism which will sense an overload on the carriage, as the carriage is elevated, and terminate operation of the drive system. However, the conventional overload protection mechanism only senses an overload when the carriage is moved upwardly and will not sense an overload when the carriage is at an upper level and is moved downwardly.
The conventional vertical conveyor is dependent solely on the lift mechanism to support the carriage at the upper level. Because of the natural elasticity of either the cable or chain, which connects the drive unit to the carriage, and due to the flexing of the hydraulic hoses in a hydraulically operated conveyor, the carriage may move as such as an inch and one half as it is loaded or unloaded at the upper level. To deal with this problem, many hydraulically operated vertical conveyors incorporate a pressure switch to stop operation of the hydraulic pump motor when the carriage is at its upper level. The pressure switch builds pressure within the hydraulic system, thereby significantly reducing the stretch of the lifting cables or chains and the flexing of the hydraulic hose. However, the pressure which is built up through use of the pressure switch bleeds off in a relatively short period of time, either because of oil leaking past the hydraulic cylinder seals or the oil leaking through the check valve, so that after the pressure is bled off, the floating condition will reappear.
A second problem encountered with the use of a pressure switch is that the switch must be set to deal with the maximum load to be placed upon the carriage. This means that the actuation of the switch occurs only when the system reaches full pressure and this full pressure condition will increase the wear on the lifting system.
It has been recognized that the unintentional descent of the carriage from an upper level can create a severe safety hazard. Unintentional descent of the carriage can be the result of overloading the carriage, either intentionally or inadvertently, beyond its rated capacity. An overload condition can occur when heavy material handling equipment, such as a fork lift truck, is run onto the carriage. If, for example, a fork lift truck, in placing a load onto the elevated carriage, drives onto the carriage, the load can be increased by a factor of two or three.
Unintentional descent of the carriage from an upper level can also occur as a result of inadequate maintenance. While most vertical conveyors require little day-to-day maintenance, situations may occur where the conveyor may be given no maintenance, with the result that components may fail, resulting in an unintentional descent of the carriage.
A further cause of unintentional descent may result from repair or maintenance work being done on the conveyor, in which the carriage at the upper or second level is used as a maintenance platform. If, during the repair or maintenance, a critical part of the unit is removed, the platform could descent unintentionally resulting in injury to the maintenance personnel.
In the past, attempts have been made to include a safety mechanism with a vertical conveyor to provide a degree of protection against uncontrolled descent of the carriage. In general, these prior mechanisms sense either excess descent speed, or failure of the lifting mechanism. The limitation of the speed sensing mechanisms is that they do not "fail safe", and their function can only be assured through regular testing. The safety mechanisms that sense failure of the lift mechanism are generally responsive to breakage of the cable or chain, which connects the drive unit to the carriage. The connecting cable or chain normally passes over a series of sheaves to provide a mechanical advantage and if the rupture of the cable or chain occurs a substantial distance from the carriage, a considerable length of cable or chain must play out before the safety mechanism is actuated.